Origin and evolution of ore fluids in the late Mesozoic Naozhi epithermal Au–Cu deposit, Yanbian area, Northeast China: evidence from fluid inclusion and isotopic geochemistry

Abstract

The Naozhi Au-Cu deposit is located on the continental margin of Northeast China, forming part of the West Pacific porphyry-epithermal gold-copper metallogenic belt. In this paper, we systematically analyzed the compositions, homogenization temperatures, and salinity of fluid inclusions as well as their noble gas isotopic and Pb isotopic compositions from the deposit. These new data show that (1) five types of fluid inclusions were identified as pure gas inclusions (V-type), pure liquid inclusions (L-type), gas-liquid two-phase inclusions (W-type, as the main fluid inclusions (FIs)), CO 2 -bearing inclusions (C-type), and daughter-mineral-bearing polyphase inclusions (S-type); (2) W-type FIs in quartz crystals of early, main, and late stage are homogenized at temperatures of 324.7-406.7, 230-338.8, and 154.6-308°C, with salinities of 2.40-7.01 wt% NaCl eq , 1.73-9.47 wt% NaCl eq , and 6.29 wt% NaCl eq , respectively. S-type FIs in quartz crystals of early stage are homogenized at temperatures of 328.6-400°C, with salinities of 39.96-46.00 wt% NaCl eq ; (3) Raman analysis results reveal that the vapor compositions of early ore-forming fluids consisted of CO 2 and H 2 O, with H 2 O gradually increasing and CO 2 being absent at the late mineralization stage; (4) fluid inclusions in pyrite and chalcopyrite have 3 He/ 4 He ratios of 0.03-0.104 Ra, 20 Ne/ 22 Ne ratios of 9.817-9.960, and 40 Ar/ 36 Ar ratios of 324-349. These results indicate that the percentage of radiogenic 40 Ar* in fluid inclusions varies from 8.8 to 15.5 %, containing 84.5-91.2 % atmospheric 40 Ar; (5) the 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 206 Pb/ 204 Pb ratios of sulfides are 18. 1822-18.3979, 15.5215-15.5998, and 38.1313-38.3786, respectively. These data combined with stable isotope data and the chronology of diagenesis and metallogenesis enable us suppose that the ore-forming fluids originated from the melting of the lower crust, caused by the subduction of an oceanic slab, whereas the mineralized fluids were exsolved from the late crystallization stage and subsequently contaminated by crustal materials/fluids during ascent, including meteoric water, and the mineral precipitation occurred at a shallow crustal level.